The present invention generally relates to the field of piezoelectric devices and more particularly, but not by way of limitation, to an annular rotary piezoelectric actuator suitable for use as a secondary fine actuator in a dual stage head positioning servo system of a hard disk drive and to a dual stage actuator system for a high density hard disk drive using the annular rotary piezoelectric actuator.
Piezoelectric actuators have been used as positioners or driving motors in a broad spectrum of fields such as optics, precision machining, fluid control and optical disk drives due to their characteristics of small size, simple structure, quick response and, most importantly, controllable displacement down to nanometers.
In the area of hard disk drives, however, there exists a competition between micro machining electrostatic, electromaganic micro actuators and piezoelectric actuators. The micro machining actuators are designed to drive the slider directly. An advantage of this type of actuator is its higher resonance frequency, but its stroke/voltage sensitivity is very small. The piezoelectric actuators, on the other hand, are commonly used to control the motion of the suspension. Compared with the former, the latter has a larger stroke/voltage sensitivity and a relatively lower resonance frequency. Unfortunately, the resonance frequency and stroke are of the same importance to the dual stage head positioning servo system of the hard disk drive. A piezoelectric actuator is disclosed in U.S. patent application Ser. No. 08/874,814 (U.S. Pat. No. 5,898,544) filed on Jun. 13, 1997 by Todd A. Krinke et al. entitled Base Plate-mounted Microactuator for a Suspension which is assigned to Hutchinson Technology Incorporated.
Regarding the piezoelectric actuator, for the purpose of decreasing the driving voltage, d31 type multilayer, split-morph multilayer and II-beam multilayer are presented as the secondary fine actuators of the dual stage servo system. However, the conflict of resonance frequency and stroke still remains unresolved.
In the present invention a micro machining piezoelectric actuator used to drive the slider directly is proposed to meet the requirements of resonance frequency and stroke simultaneously.
A general object of the present invention is to provide a new structure of piezoelectric actuator which has a function of generating rotary deformation around its shape center.
A specific object of the present invention is to provide a piezoelectric actuator which can be used as a secondary actuator in a dual stage servo system of a hard disk drive.
A more specific object of the present invention is to provide a piezoelectric actuator which can be mounted between the suspension and the slider to drive the magnetic pole tip of the slider to move across data tracks so as to obtain a head positioning servo system with high bandwidth.
Yet another specific object of the present invention is to provide a piezoelectric actuator which has a symmetric configuration and a simple potting process so that it can be easily manufactured and realized in a head gimble assembly (HGA).
Another object of the present invention is to provide a piezoelectric actuator which has enough displacement stroke to meet the requirement of compensating tracking misregistration under a supply voltage up to 15 volts.
The above objects can be achieved according to embodiments of the present invention by designing the configuration, selecting appropriate electrode patterns and corresponding poling scheme of a piezoelectric element. Hereinafter the typical practice means will be described.
In accordance with one aspect of the present invention, an annular piezoelectric element with a gap along the radial direction is divided into two parts by an electrode crevice along its circumference at a certain radius. Polarization vectors in these parts are generally in the direction of its thickness and can be polarized either in the same or opposite direction. One of the two ends of the element is affixed to a base. Driving voltages are so arranged that they make one of the two parts expanded by the xe2x80x9831xe2x80x99 action while the other contracted, or one of the two parts expanded or contracted while the other remains constant. This will result in roughly a rotary motion of the free end around its center, rather like the bending of a split-morph actuator, where the bending is in the direction of its width. This actuator is actually an annular split electrode rotary piezoelectric actuator. Analogous to the name of xe2x80x9csplit-morphxe2x80x9d, it can be defined as xe2x80x9cannular split-morphxe2x80x9d. If only one of the two parts is polarized, this actuator can be called partial poling annular split-morph. If the poling vectors in the two corresponding parts of the element are opposite to each other, this actuator is called antiparallel annular split-morph, and if the poling vectors are the same, it is called parallel annular split-morph.
In accordance with another aspect of the present invention, a dual stage head positioning actuator system is provided for a hard disk drive having a plurality of disks and a plurality of vertically aligned head sliders mounted on distal ends of a plurality of suspensions via micro piezoelectric actuators. In one embodiment, the dual stage head positioning actuator system includes
a voice coil motor as the primary stage actuator to simultaneously drive the plurality of suspensions in a long stroke; and
a plurality of piezoelectric actuators as the secondary fine actuator to drive the head slider individually in a fine stroke for precisely positioning a slider to a predetermined position on a respective disk surface of the plurality of disks.
The actuator as described above is used as the fine actuator and each of the plurality of fine actuators is mounted on an associated flexure tongue of one of the plurality of suspensions through the base, and the slider is potted on the inner disc of the actuator.
In accordance with a further aspect of the present invention, a driving voltage scheme is provided for any two pieces of the piezoelectric elements or any two active parts in one piezoelectric element that enable the two pieces or two parts to expand and contract at the same time, respectively, while the direction of the driving voltages preserves the poling directions of the corresponding pieces or parts throughout the operation so as to prevent depoling of the piezoelectric element. The voltages applied on the electrodes of the two pieces or two parts are two opposing phase AC signals added to a positive DC bias which has the same directions as the poling vector of the pieces or parts, the bias is larger than or at least equal to the amplitude of the AC signal.
Consequently, the driving voltages can precisely control the rotary angle of the free end of the suspension.
One important advantage and a novel aspect of the present invention involves the feature of the annular structure and the method of separating the annular structure into active parts and inactive parts which enable the actuator to have the function of generating rotary motion within the electrode plane. With these advantages and features, the actuator with dimensions similar to the slider can be installed between the suspension, more specifically, flexure tongue and slider to drive the magnetic pole tip of the slider wide and fast to follow a moderate runout.
Another important feature of the present invention is that the actuator can easily be fabricated into a multilayer structure with various dimensions by tape casting, thick film screen printing, sand blasting and techniques of MEMS (microelectro mechanical systems).
Other objects, as well as the structure and features of the present invention to achieve those objects, will be apparent by considering the following detailed description of preferred embodiments, presented in conjunction with the accompanying drawings.